1. Field of the Invention
This invention relates to a novel power converting device and a novel protection device for the power converting device.
2. Description of the Prior Art
A variety of circuit systems are employed as the power converting devices which are connected with this invention. One example is shown in FIG. 1. In this Figure, 11 is an A.C. terminal, 12 is a forward rectifier, 13 is a reverse rectifier, 14 and 15 are D.C. terminals, and 16 indicates thyristors which act as the power converting elements. It is well known that a circuit composed as shown in FIG. 1 can convert A.C. power fed in from the A.C. terminal 11 into D.C. power by means of the forward rectifier 12; and that it can convert D.C. power fed in from the D.C. terminals 14 and 15 into A.C. power by means of the reverse rectifier 13. In such a case, when either the forward rectifier 12 or the reverse rectifier 13 is operating, it is necessary that the other rectifier be completely inoperative. If the rectifier is not totally inoperative, D.C. and A.C. short circuiting occurs. In order to prevent this, a technique has been employed to allow a prescribed time for the change-over between operating the forward rectifier 12 and the reverse rectifier 13. Another method is set forth in Japanese Patent Publication No. 45-40969. In order to gain a more rapidly responding control, it is necessary to make the above-mentioned operational change-over time as short as possible. For this, the method set forth in Japanese Patent Publication No. 45-40969 is excellent. In this method, for example, the voltages across thyristors 16 in the forward rectifier 12 are detected during operation and when sufficient reverse voltage to be applied to these thyristors 16 is detected, the thyristors 16 of the reverse rectifier 13 are fired.
The detection method which is set forth in Japanese Patent Publication No. 45-40969 will now be explained by means of FIG. 2. FIG. 2(a) shows an example of detecting the voltage of a thyristor 16, while FIG. 2(b) shows the waveforms of the thyristor voltage .sup.e A-K and current .sup.i A when the thyristor 16 is extinguished. As is shown in FIG. 2(b), the current .sup.i A decreases and becomes zero at time t.sub.1, after which the reverse current reaches a peak at a time t.sub.2. As FIG. 2(b) also shows, the thyristor voltage .sup.e A-K detected at the detection terminals 17 in FIG. 2(a) reveals that the reverse voltage begins to be applied to the thyristor 16 after time t.sub.2, when the reverse current reaches a peak. At time t.sub.3 the reverse current becomes zero, and at time t.sub.4 the period of reverse voltage is completed. Since the thyristor 16 cannot be fully extinguished if a reverse voltage is not applied for a period of several tens of microseconds to several hundreds of microseconds, it can be judged whether or not the thyristor 16 has been fully extinguished, by keeping track of the reverse voltage of the thyristor 16 at the voltage detection terminals 17. Thus a high reliability, rapid-response power converting device can be offered, in which the other rectifier (either the forward rectifier 12 or the reverse rectifier 13) can be brought on line when it is ascertained that the thyristors 16 are completely extinguished.
However, although this method can easily be put into practice when the thyristors 16 have a low circuit voltage, if the circuit voltage reaches several thousand volts, then it becomes difficult to isolate the voltage detection terminals 17 and the circuit which judges the reverse voltage period (not illustrated). Consequently, a drawback of this method is that it is difficult to put into practice in high-voltage power converting devices.
Another problematic point in existing systems is that it is the magnitude of the reverse voltage applied to the thyristor 16 which is utilized to determine whether or not sufficient reverse voltage has been applied to the thyristor 16. Thus the method which has been adopted is to make the determination at time t.sub.31 in FIG. 2(b), whether or not the reverse voltage e.sub.A-K is in accordance with a reference reverse voltage E.sub.1, rather than to wait until the time t.sub.4 to determine whether or not the reverse voltage has in fact been sufficient. In such a case the interval from time t.sub.2 to time t.sub.31 is usually set to be greater than the turn-off time required by the thyristor 16. The resulting defect has been that, due to the interval of reverse voltage application from time t.sub.31 to time t.sub.4 not being properly detected and evaluated, the protection circuit operates erroneously, unless a larger reverse voltage than that required to turn off the thyristor 16 is applied to the thyristor 16.
Generally, the reverse voltage applied to the thyristor in the power converting device changes dependently upon the changes of the voltage applied to the thyristor by the power source. In the power converting devices which are utilized in variable voltage-variable frequency power supplies, the voltage varies approximately in proportion to the frequency. In such devices, in the low-frequency operation region, the magnitude of the reverse voltage itself is small. For example, if the magnitude of the frequency is set to be 20% of that shown in FIG. 2(b), the magnitude of the reverse voltage applied to the thyristor 16 becomes 20% of the voltage e.sub.A-K shown in FIG. 2(b). In this case, the thyristor voltage e.sub.A-K even at time t.sub.2, is smaller than the reference reverse voltage E.sub.1 in FIG. 2(b). As a result, in the power converting devices which are utilized in variable voltage-variable frequency power supplies, erroneous operation of the protection device has occurred.